High density polyethylene (HDPE) is an important commercial product, large quantities being produced worldwide. HDPE is typically recognized (and is defined for the purposes of the present invention) as a substantially linear, semi-crystalline, polymer of ethylene (preferably a homopolymer but also on occasion containing very minor amounts of other well-known comonomers), possessing a density of 0.94 g/mL or higher.
An important use of HDPE is in packaging, which may be divided into two general types—rigid packaging such as bottles and tanks, and flexible packaging such as bags and pouches. The former may be formed by such methods as blow or injection molding, and the latter are usually formed from films having one or more layers, at least one of which is HDPE.
HDPE is a favored packaging material for many products because of low cost, relatively easy formability and good toughness, and for some products having low permeation rates for certain materials either deleterious to these products, or to keep the package's contents from diffusing from the package and being lost, such as water and/or oxygen. Among the types of products where these low permeation rates are important are foods, both dry and liquid materials, and lubricating oils. For example for dry foods low water vapor transmission rates are important to keep the foods crisp, while low oxygen transmission rates are important for any foods that may oxidize, forming off colors and/or tastes or smells in the food. The lower the transmission rates of the packaging, the better the food will taste and/or look, and/or the longer the food may be stored before being used, and/or the thickness of the packaging may be reduced without deleteriously affecting absolute rates of transmission, all of course important advantages. In some instances, such as bottles for toiletries such as perfume or cologne, it may be desirable to keep water in and/or oxygen out. Other combinations will be obvious to the artisan.
U.S. Pat. No. 5,955,555, WO99/12981, WO99/46302, WO99/46303, WO99/46304, WO99/46308, WO99/62963 (corresponding to U.S. patent application Ser. No. 09/317,104, filed 21 May 1999), WO00/15646, WO00/24788, WO00/32641, G. J. P. Britovesk, et al., J. Chem. Soc., Chem. Commun., p. 849-850 (1998), and B. L. Small, et al., Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) vol. 39, p. 213 (1998) (all incorporated by reference herein for all purposes as if fully set forth), all report the polymerization of ethylene using iron and cobalt complexes of certain tridentate ligands. No mention is made of the use of the resulting polymers for packaging where improved (lower) water vapor and/or oxygen permeation rates of HPDE are of interest.